1. Field of the Invention
This invention relates to an electronic device modeling the temperature rise in a motor, making use of current sensors placed on the motor power supply phases.
2. Discussion of the Background
Temperature rises dangerous for a motor may be caused by a low prolonged overload, a slight phase unbalance, excessively long or frequent startups, the motor getting unable to turn, or intermittent conditions. An electronic protection relay takes action by calculating the motor temperature starting from signals supplied by sensors located on each of the power supply phases. This calculation is done taking account of thermal inertia.
The electronic relay described in patent U.S. Pat. No. 3,808,503 comprises a current sensor on each motor power supply phase sending an "image" signal of the phase current to an electronic processing circuit that outputs a current proportional to the square of the motor current, to a circuit modeling the motor temperature rise consisting of an R-C circuit.